Repetition Statements

5 Repetition Statements

Contents • Basic loop structures • while loops • Interactive while loops • for loops • Loop programming techniques • Nested loops • dowhile loops • Common programming errors

Basic Loop Structures • Repetition structure has four required elements: • Repetition statement • Condition to be evaluated • Initial value for the condition • Loop termination • Repetition statements include: • while • for • do while

Basic Loop Structures • The condition can be tested • At the beginning: Pretest or entrance-controlled loop • At the end: Posttest or exit-controlled loop • Something in the loop body must cause the condition to change, to avoid an infinite loop, which never terminates

Pretest and Posttest Loops • Pretest loop: Condition is tested first; if false, statements in the loop body are never executed • while and for loops are pretest loops Figure 5.1 A pretest loop

Pretest and Posttest Loops • Posttest loop: Condition is tested after the loop body statements are executed; loop body always executes at least once • do whileis a posttest loop Figure 5.2 A posttest loop

Fixed-Count Versus Variable-Condition Loops • Fixed-count loop: Loop is processed for a fixed number of repetitions • Variable-condition loop: Number of repetitions depends on the value of a variable

while Loops • while statement is used to create a while loop • Syntax: • while (expression) • statement; • Statements following the expressions are executed as long as the expression condition remains true (evaluates to a non-zero value)

while Loops • Program 5.1 • #include <iostream> • using namespace std; • int main() • { • int count; • count = 1; // initialize count • while (count <= 10) • { • cout << count << " "; • count++; // increment count • } • return 0; • } • The Output from Program 5.1 • 1 2 3 4 5 6 7 8 9 10

while Loops • Program 5.3 • #include <iostream> • #include <iomanip> • using namespace std; • int main() • { • int num; • cout << "NUMBER SQUARE CUBE\n" • << "------ ------ ----\n"; • num = 1; • while (num < 11) • { • cout << setw(3) << num << " " • << setw(3) << num * num << " " • << setw(4) << num * num * num << endl; • num++; // increment num • } • return 0; • }

while Loops • The Output from Program 5.3 • NUMBER SQUARE CUBE • ------ ------ ---- • 1 1 1 • 2 4 8 • 3 9 27 • 4 16 64 • 5 25 125 • 6 36 216 • 7 49 343 • 8 64 512 • 9 81 729 • 10 100 1000

while Loops • Example: • Celsius-to-Fahrenheit temperature-conversion • Display Fahrenheit and Celsius temperatures, from 5-50 degrees C, in 5 degree increments celsius = 5; // starting Celsius value while (celsius <= 50) { fahren = (9.0/5.0) * celsius + 32.0; cout << setw(4) << celsius << setw(13) << fahren << endl; celsius = celsius + 5; }

Interactive while Loops • Combining interactive data entry with the whilestatement provides for repetitive entry and accumulation of totals

Interactive while Loops Figure 5.6 Accumulation flow of control

Interactive while Loops • Program 5.6 • #include <iostream> • using namespace std; • int main() • { • const int MAXNUMS = 4; • int count; • double num, total; • cout << "\nThis program will ask you to enter " • << MAXNUMS << " numbers.\n"; • count = 1; • total = 0 • while (count <= MAXNUMS) • { • cout << "\nEnter a number: "; • cin >> num; • total = total + num; • cout << "The total is now " << total; • count++; • } • cout << “\n\nThe final total is “ << total << endl; • return 0; • }

Interactive while Loops Sample run of program 5.6: This program will ask you to enter 4 numbers. Enter a number: 26.2 The total is now 26.2 Enter a number: 5 The total is now 31.2 Enter a number: 103.456 The total is now 134.656 Enter a number: 1267.89 The total is now 1402.546 The final total is 1402.546

Sentinels • Sentinel: A data value used to signal either the start or end of a data series • Use a sentinel when you don’t know how many values need to be entered

break and continue Statements • break statement • Forces an immediate break, or exit, from switch,while,for, and do-while statements • Violates pure structured programming, but is useful for breaking out of loops when an unusual condition is detected

break and continue Statements • Example of a break statement: while( count <= 10) { cout << “Enter a number: ”; cin >> num; if (num > 76) { cout << “You lose!\n”; break; // break out of the loop } else cout << “Keep on tracking!\n”; count++; } // break jumps to here

break and continue Statements • A continue statement where invalid grades are ignored, and only valid grades are added to the total: while( count < 30) { cout << “Enter a grade: ”; cin >> grade; if (grade < 0 || grade > 100) continue; total = total + grade; count++; }

break and continue Statements • continue statement • Applies to while,do-while, and for statements; causes the next iteration of the loop to begin immediately • Useful for skipping over data that should not be processed in this iteration, while staying within the loop

The NullStatement • Null statement • Semicolon with nothing preceding it • ; • Do-nothing statement required for syntax purposes only

for Loops • for statement: A loop with a fixed count condition that handles alteration of the condition • Syntax: • for (initializing list; expression; altering list) • statement; • Initializing list: Sets the starting value of a counter • Expression: Contains the maximum or minimum value the counter can have; determines when the loop is finished

for Loops • Altering list: Provides the increment value that is added or subtracted from the counter in each iteration of the loop • If initializing list is missing, the counter initial value must be provided prior to entering the for loop • If altering list is missing, the counter must be altered in the loop body • Omitting the expression will result in an infinite loop

for Loops • Program 5.9 • #include <iostream> • #include <iomanip> • #include <cmath> • using namespace std; • int main() • { • const int MAXCOUNT = 5; • int count; • cout << "NUMBER SQUARE ROOT\n"; • cout << "------ -----------\n"; • cout << setiosflags(ios::showpoint); • for (count = 1; count <= MAXCOUNT; count++) • cout << setw(4) << count • << setw(15) << sqrt(double(count)) << endl; • return 0; • }

for Loops Figure 5.7for loop flowchart.

for Loops • Program 5.11 • #include <iostream> • #include <iomanip> • using namespace std; • int main() • { • const int MAXNUMS = 10; • int num; • cout << endl; // print a blank line • cout << "NUMBER SQUARE CUBE\n" • << "------ ------ ----\n"; • for (num = 1; num <= MAXNUMS; num++) • cout << setw(3) << num << " " • << setw(3) << num * num << " " • << setw(4) << num * num * num << endl; • return 0; • }

for Loops When Program 5.11 is run, the display produced is NUMBER SQUARE CUBE _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 1 1 2 4 8 3 9 27 4 16 64 5 25 125 6 36 216 7 49 343 8 64 512 9 81 729 10 100 1000

A Closer Look: Loop Programming Techniques • These techniques are suitable for pretest loops (for and while): • Interactive input within a loop • Includes a cin statement within a while or for loop • Selection within a loop • Using a for or while loop to cycle through a set of values to select those values that meet some criteria

A Closer Look: Loop Programming Techniques • Program 5.13 • #include <iostream> • using namespace std; • // this program computes the positive and negative sums of a set • // of MAXNUMS user entered numbers • int main() • { • const int MAXNUMS = 5; • inti; • double usenum, postot, negtot; • postot = 0; // this initialization can be done in the declaration • negtot = 0; // this initialization can be done in the declaration • for (i = 1; i <= MAXNUMS; i++) • { • cout << "Enter a number (positive or negative) : "; • cin >> usenum; • if (usenum > 0) • postot = postot + usenum; • else • negtot = negtot + usenum; • } • cout << "The positive total is " << postot << endl; • cout << "The negative total is " << negtot << endl; • return 0; • }

A Closer Look: Loop Programming Techniques • Evaluating functions of one variable • Used for functions that must be evaluated over a range of values • Noninteger increment values can be used

A Closer Look: Loop Programming Techniques • Program 5.14 • #include <iostream> • #include <iomanip> • #include <cmath> • using namespace std; • int main() • { • int x, y; • cout << "x value y value\n" • << "------- --------\n"; • for (x = 2; x <= 6; x++) • { • y = 10 * pow(x,2.0) + 3 * x - 2; • cout << setw(4) << x << setw(11) << y << endl; • } • return 0; • }

A Closer Look: Loop Programming Techniques • Interactive loop control • Variable is used to control the loop repetitions • Provides more flexibility at run-time

A Closer Look: Loop Programming Techniques • Program 5.16 • #include <iostream> • #include <iomanip> • using namespace std; • // this program displays a table of numbers, their squares and cubes • // starting from the number 1. The final number in the table is • // input by the user • int main() • { • int num, final; • cout << "Enter the final number for the table: "; • cin >> final; • cout << "NUMBER SQUARE CUBE\n"; • cout << "------ ------ ----\n"; • for (num = 1; num <= final; num++) • cout << setw(3) << num • << setw(8) << num*num • << setw(7) << num*num*num << endl; • return 0; • }

Nested Loops • Nested loop: A loop contained within another loop • All statements of the inner loop must be completely contained within the outer loop; no overlap allowed • Different variables must be used to control each loop • For each single iteration of the outer loop, the inner loop runs through all of its iterations

Nested Loops Figure 5.9 For each i, j loops.

Nested Loops • Program 5.16 • #include <iostream> • using namespace std; • int main() • { • const int MAXI = 5; • const int MAXJ = 4; • inti, j; • for(i = 1; i <= MAXI; i++) // start of outer loop <------+ • { // | • cout << "\ni is now " << i << endl; // | • // | • for(j = 1; j <= MAXJ; j++) // start of inner loop | • cout << " j = " << j; // end of inner loop | • } // end of outer loop <-------+ • cout << endl; • return 0; • }

do while Loops • do while loop is a posttest loop • Loop continues while the condition is true • Condition is tested at the end of the loop • Syntax: do statement; while (expression); • All statements are executed at least once in a posttest loop

do while Loops Figure 5.10 The do while loop structure.

do while Loops Figure 5.11 The do statement’s flow of control.

Validity Checks • Useful in filtering user-entered input and providing data validation checks • Can enhance with if-else statement do { cout << “\nEnter an identification number: ”; cin >> id_num; } while( id_num < 1000 || id_num > 1999 );

Common Programming Errors • Making the “off by one” error: loop executes one too many or one too few times • Using the assignment operator (=) instead of the equality comparison operator (==) in the condition expression • Testing for equality with floating-point or double-precision operands; use an epsilon value instead

Common Programming Errors • Placing a semicolon at the end of the for clause, which produces a null loop body • Using commas instead of semicolons to separate items in the for statement • Changing the value of the control variable • Omitting the final semicolon in a do statement

Summary • Loop: A section of repeating code, whose repetitions are controlled by testing a condition • Three types of loops: • while • for • do while • Pretest loop: Condition is tested at beginning of loop; loop body may not ever execute; ex., while, forloops

Summary • Posttest loop: Condition is tested at end of loop; loop body executes at least once; ex., do while • Fixed-count loop: Number of repetitions is set in the loop condition • Variable-condition loop: Number of repetitions is controlled by the value of a variable

Repetition Statements